1. Field of the Invention
Embodiments of the present invention generally relate to olefin polymerization catalysts. More particularly, embodiments of the present invention generally relate a catalyst system containing a metal atom bound to at least two Group 15 atoms and their use in gas or slurry phase to produce polyolefins.
2. Description of the Related Art
Advances in polymerization and catalysis have produced new polymers having improved physical and mechanical properties useful in a wide variety of products and applications. With the development of new catalysts, the choice of polymerization, such as solution, slurry, high pressure or gas phase, for producing a particular polymer has been greatly expanded. Advances in polymerization technology have also provided more efficient, highly productive and economically enhanced processes.
Metallocene catalysts have been used to produce resins having desirable product properties. While these resins have excellent toughness properties, particularly dart impact properties, these resins can be difficult to process. One approach to improve the processing of such metallocene catalyzed polyethylenes has been to blend them with another polymer. While the polymer blend tends to be more processable, the blend is expensive and adds a cumbersome step to the manufacturing process.
Another approach has been to produce two polymers together at the same time in the same reactor using two different catalysts. For example, WO 99/03899 discloses using a typical metallocene catalyst and a conventional Ziegler-Natta catalyst in the same reactor to produce a bimodal MWD HDPE. Other catalysts, such as anionic, multidentate heteroatom ligands, have also been used in mixed catalyst system. For example, U.S. Pat. No. 5,576,460 describes a preparation of arylamine ligands. U.S. Pat. No. 5,889,128 discloses a process for the living polymerization of olefins using initiators having a metal atom and a ligand having two group 15 atoms and a group 16 atom or three group 15 atoms. EP 893 454 A1 describes titanium transition metal amide compounds. U.S. Pat. No. 5,318,935 discusses amido transition metal compounds and catalyst systems for producing isotactic polypropylene. U.S. Pat. No. 5,506,184 discloses polymerization catalysts containing bidentate and tridentate ligands.
Polymers produced by two different catalysts types, however, exhibit unpredictable characteristics. The polymers produced from two different catalysts can exhibit different physical and mechanical properties compared to the individual polymers produced separately from each catalyst and blends thereof. This unpredictability may occur due to competition or other influence between the catalyst or catalyst systems used. This unpredictability may also occur due to differences in solubility of the individual catalyst components, reaction kinetics of the individual catalyst components, and the rate of decay of the individual catalyst components, just to name a few. There is a need, therefore, for a combination of compatible catalysts capable of producing polyolefin polymers having desirable combinations of processing, mechanical, and optical properties.